(1) Field of the Invention
The present invention relates to bioreactors including improved liquid distribution and gas collection features. The present invention also includes wet landfills and landfills having one or more bioreactors of this invention.
(2) Description of the Art
About 15 years ago, landfill owners began using leachate recirculation as a remediation method. Generally accepted techniques are to inject leachate into landfills using horizontal leachate injection piping, vertical wells, or by applying leachate to the working landfill face surface. Distributing leachate over a large area has always been a problem. Often the leachate distribution piping becomes fouled with slime, fines or calcium/iron complexes from landfill material surrounding the leachate injection sites.
Typical landfills also include piping and wells for recovering valuable landfill gases that contains methane. Over 90% of the landfill gas recovery systems in the United States and Canada use vertical gas extraction wells. Standard practice is to bore a 30″ to 36″ diameter well into a landfill, insert a 6″ to 8″ diameter perforated gas extraction pipe into the well and then fill the well with stone. One problem with vertical gas extraction wells is that the gas pipes fill with water and recirculated leachate. If the water is not removed, the gas extraction well becomes useless. The standard method of removing water from vertical gas extraction wells is to associate expensive pumps with the gas extraction wells to keep the wells free of liquids.
Another problem with landfill remediation is how to accelerate biological degradation and increase gas production. One way to accelerate landfill biodegradation is to ensure that the landfill moisture content remains high and/or to inject air into the landfill. In an effort to enhance landfill waste biodegradation rates, landfill operators in the United States have been actively recirculating leachate and pursuing bioreactor landfill practices that utilize the addition of liquids from sources outside of the landfill to increase the moisture content of the waste. The preferred waste moisture content range of these advanced sanitary waste cells is 35% to 45% (wet weight basis). With the onset of over 150 leachate recirculation projects in the U.S. and over 20 bioreactor landfills, an increase in maintenance costs in the vertical well fields and a potential for increased surface emissions have created problems. Classic vertical gas extraction wells fill with leachate and become useless. This has resulted in the need for the installation of automatic pump out systems and an increased frequency of maintenance to reduce surface emissions.
Several advances in landfill remediation have been recently made. U.S. Pat. No. 6,742,962 discloses a horizontal infiltration and gas recovery system. The system is designed to eliminate some of these problems with gas recovery in wet landfill systems. While the system is an improvement over vertical gas recovery systems it still requires liquid pumps associated with the gas recovery wells and the system is subject to oxygen intrusion if the landfill cover soil is not properly compacted. Another issue with this system is the potential for limited liquid coverage due to non-homogeneous waste placement and chemical blinding of the trenches.
There has been some work in developing landfill bioreactors that have horizontal gas extraction piping systems. Such a landfill gas collection system is described in U.S. Pat. No. 7,118,308, the specification of which is incorporated herein by reference. The '308 patent system covers an area of up to 80% of the landfill surface of a constructed lift of waste with a permeable material as illustrated in FIG. 1.
Despite these advances in landfill bioremediation, there remain problems with existing leachate recirculation and/or landfill gas recovery systems including (1) the continued dependency on vertical gas wells; (2) the requirement to pump liquids out of vertical gas wells as the infiltration field became saturated; (3) historically, infiltration systems do not allow for surface application initially followed by deep infiltration and the landfill is filled for the apparatus is covered; (4) the need for a system that would allow for infiltration and gas recovery simultaneously. Furthermore, there is a need to prevent the accumulation of and/or to drain liquids from the gas collection system. There is also a need to improve gas recovery for the saturated infiltration field. Additionally gas systems in older wet landfills are prone to maintenance issues related to settling of the gas system.